In damp atmospheres , you get electromigration , i.e. metal ions from the positive terminal will migrate over to the negative terminal. The positive wire will get smaller and smaller, and the negative wire will get "bigger" . In the special case where the negative and positive terminals are in an electroplating bath, you will get metal plated out on the negative.

In a damp marine environment, with megohms of leakage the ions migrate to the cathode, dragging whatever along with them.

If you look at aluminium in a damp or submerged situation, it will form lumps of jelly, the jelly is actually aluminium hydroxide, which is very hygroscopic, so picks up ~ 10x its mass in water , if you dry out the jelly, you get a small trace of white powder. The AlOH jelly can also pick up the smallest traces of copper and other metals, and make really bright colors.

If you look at steel submerged in still fresh water (or sometimes inside an old paint tin) the steel first tries to oxidise, but sucks up all the available oxygen (making a small trace of brown "rust") It is then forced to oxidise by making Iron Hydroxide and Iron oxide, which is black in color, these grow as little mushrooms or coral like growths. And again the mushroom volume is mostly water. Note that the coral or mushroom growths can ONLY grow where there is an electrochemical gradient, this occurs at the occasional defect or the surface of an existing deposit. So you end up with 95% of the surface clean steel, and really big mushrooms/coral growing in a few spots.

So your pretty growths may well be the result of an electrolyte leak, but the marine atmosphere and condensation may be sufficient. I'd guess the copper terminals are zinc plated, and it's the zinc that is making zinc hydroxide. The hydroxide, like most hydroxides is very hygroscopic and sucks up water to make a gel, but with a bit less water it will try to make cystals , so you end up with a jelly blob with coral like structures growing off the top. Traces of Copper hydroxides / oxides can contribute colors from black-brown-red- green- blue.

Because metal disappearing from the positive terminal is a problem , telephone systems, and most marine installations are earthed at the positive terminal. This may well have been the case with your installation, in which case the negative terminal would have sucked all sorts of metal ions to it. With only 1 nano amp of leakage , you get 6241000000 atoms of metal migrating to the negative terminal per second.

-salbayeng - that is a super explanation of the chemistry involved and how the thing 'grew'. You have also solved the mystery of the gel substance (its been bugging me for years!). A great post, many thanks for the lesson in electro-chemistry :-)